1. Field of the Invention
This invention relates to a polymeric coating for solar cells and to a polymer coated solar cell, and is particularly directed to polyimide coatings for solar cells and solar cells coated with such polyimides and which coatings are colorless and transparent, relatively non-brittle, have good thermal properties, resistance to ultraviolet radiation degradation, effective in repelling low energy protons and also have other desirable characteristics for use in outer space.
2. Description of the Prior Art
Polymeric coatings for solar cells are required to provide a high degree of solar radiation protection. Conventional polyimides have the requisite thermal stability, but they are colored, and thus absorb in the visible portion of the solar spectrum which reduces performance and generates heat which promotes degradation. Other polymers discolor excessively or lose structural integrity for various other reasons, such as instability to solar protons, electrons, or ultraviolet radiation. When these polymer coatings discolor, the portion of solar spectrum transmitted to the solar cells is reduced, thus effectively reducing the solar cell efficiency.
In the prior art, coatings such as polyurethanes, epoxies, polyesters, polyimides, etc., were evaluated and found to have inadequate stability for various reasons. Perfluorinated aliphatic hydrocarbons, such as FEP Teflon (tetrafluoroethylene hexafluoropropylene copolymer), had a better potential, but had inadequate stability. Further, it is necesssry to employ an adhesive or a fusion process in order to apply the Teflon or other polymer coating to the solar cell itself. Usually such an adhesive is a silicone. However, after a period of time, the adhesive or fusion boundary fails and the Teflon becomes separated from the solar cell, thereby accelerating deterioration and failure of the cell.
Because of the limitations of known polymers heretofore used or considered for solar cells, and particularly for solar cells positioned in outer space, it was necessary to employ a quartz cover over the solar cell itself, in order to protect the cell against low energy particles such as low energy proton bombardment. However, the quartz layer was usually quite heavy (relative to a polymeric coating). This quartz layer thus increased the overall weight and made introduction of the solar cell into very large arrays much more difficult and costly.
Some of the commonly used protective covers for solar cells are formed of fused silica. These fused silica layers usually require a multi-layer filter to effectively filter out all ultraviolet radiation, as for example, that radiation less than 0.35 .mu.m, to protect the adhesive (e.g., a silicone adhesive) from darkening and losing transmission and hence power. Another protective cover commonly used is Ceria, namely a cerium oxide doped glass micro-sheet. This glass sheet has a natural cut-on frequency of about 0.35 .mu.m and hence does not require a multi-layer filter for adhesive protection. The Ceria glass sheet does provide for radiation stability.
In addition to the foregoing, a fused silica without a multi-layer filter has been used with the FEP Teflon as the adhesive. This combination is quite effective in view of the fact that glass processing is minimized. However, the overall covered solar cell is still too heavy for use as a space cell in extremely large solar cell arrays.
The method of protecting solar cells which is currently used to a large degree involves the adhesive bonding of a thin ZnO/Ta.sub.2 O.sub.5 coated quartz coverslide onto the solar cells with a silicone adhesive. The multi-layer ZnO/Ta.sub.2 O.sub.5 coating on the quartz is a solar radiation cut-off coating which provides damaging solar radiation (approximately less than 0.32 .mu.m) from penetrating through the quartz into the silicone adhesive. Although this means of protecting solar cells is effective, it is too expensive and impractical to apply to massive solar arrays, such as those which would be required for capturing solar radiation on a massive scale in space and transmitting it to earth. This technique is practical only for relatively "small" devices such as communication satellites where the added costs can be tolerated. It is not practical for large size applications, e.g., acre size applications.
Consequently, there still exists a need for a much more reliable, light weight, protective coating for solar cells, and particularly for those solar cells which are to be exposed to outer space environments. The prior art cover materials or coating materials such as quartz and the other polymers, are of limited effectiveness. The polymer systems, particularly, suffer from the disadvantge in that they exhibit loss of transmission, cracking and loss of adhesion to the cell after exposure to solar ultraviolet radiation for any reasonable period of time. Further, adhesion is affected materially after subjection to the outer space environments. Consequently, the critical properties for solar cell coatings include the property of being resistant to ultraviolet radiation and maintaining optical clarity, even after a period of time. These coatings must also be resistant to degradation by low energy protons and exhibit good adhesion to the cell after some thermal cycling. Further, they must be formulated for ease of application to the solar cells without any damage to the cell itself. In addition, the coatings must be formulated to be more cost effective than the previously and presently used quartz and glass systems.
U.S. Pat. No. 3,356,648 discloses linear polyimides derived from the hexafluoroisopropylidene bridged diamine, such as dianiline, and tetracarboxylic dianhydride. These polyimides are useful as shaped structures and for wrappings, packaging and the like.
U.S. Pat. No. 3,959,350 discloses linear polyimides similar to those of U.S. Pat. No. 3,356,648, but which do not have hexafluoroisopropylidene groups bridging the amine moieties. These polyimides are useful as molding compounds, and for preparing self-supporting film structures and composites. However, these polyimides are not colorless. The lack of transparency and the other physical characteristics, such as processability of these ppolyimides, render them ineffective as solar cell coatings.